The present invention relates to integrated circuits and, more particularly, to forming implanted regions in CMOS (complementary metal oxide semiconductor) image sensors.
Digital cameras are often provided with digital image sensors such as CMOS image sensors. Digital cameras may be stand-alone devices or may be included in electronic devices such as cellular telephones or computers. A typical CMOS image sensor has an array of image sensor pixels containing contain thousands or millions of pixels. Each pixel might have a photosensitive element such as a photodiode. Photodiodes are formed in a substrate. Isolation regions may be formed in the substrate between photodiodes to reduce crosstalk between photodiodes. Isolation regions may be formed using ion implantation.
To improve image quality, it is often desirable to increase the number and density of pixels on an image sensor. The density of pixels can be represented by a quantity called “pixel pitch,” in which higher pixel pitches represent lower pixel densities and bigger pixel sizes. Each pixel can contain one or more photosensitive elements such as photodiodes. Photodiodes can be separated by isolation regions formed by ion implantation. The percentage of pixel area occupied by photodiode may be referred to as a fill factor. As pixel sizes are reduced, it can become important to decrease the area occupied by isolations regions in order to maintain or maximize the photodiode fill factor. As pixel pitches are decreased, photodiodes may need to be formed deeper in a substrate to avoid loss of sensitivity. Deeper photodiodes may require deeper isolation regions.
When conventional methods are used for implanting isolation regions, it can be challenging to form very narrow and deep isolation regions. Isolation regions are typically formed by implanting ions through the openings of patterned photoresist. If very narrow openings are formed in photoresist, the walls of the openings may be unstable. Narrower openings could be formed in thinner photoresist, but thinner photoresist would limit the permissible energies used during ion implantation and resulting implants would be too shallow. Shallow isolation regions are undesirable because they would limit photodiode depth, reducing the quantum efficiency and sensitivity of the pixels.
It would be therefore be desirable to have improved methods for forming implanted regions.